Challenges of Measuring Canopy Stomatal Conductance at Lower Wind Speeds.

The Crop Water Stress Index has long been used to determine water stress in plants, but it is an empirical, relative index. As an improved replacement, we have begun continuous measurement of canopy stomatal conductance of potatoes in Minnesota, and alfalfa in Utah for use in irrigation scheduling. At the ASA meetings last year we showed that measurements of plant water status based on canopy temperature were considerably more difficult to make in cloudy, cool, and humid conditions. Our measurements and models also indicate that these measurements are more difficult to make when the wind speed is below approximately 1 meter per second. There are two reasons for the challenges at low wind speeds: 1) standard cup anemometers are much less accurate below 1 m per second, and 2) boundary layer conductance models are less accurate when thermally-induced convective forces become a significant portion of the total boundary layer heat conductance. We will discuss the relative contribution of these components in determining canopy stomatal conductance at lower wind speeds.